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Name: Allison
Status: student
Grade: 9-12
Location: PA
Country: USA
Date: Fall 2012


Question:
I am trying to learn how air resistance might affect a rolling ball, say, a marble, but most of what I find deals with projectiles. Does air resistance on an object change if the object is on a track instead of in the air? Do the factors affecting air resistance on projectiles affect air resistance on a ball in a track in the same way? (Main factors being velocity squared and size and shape of the object.)



Replies:
Hi Allison,

I think that you have almost answered your own question! You stated (correctly) that air resistance encountered by a moving object, increases as the (approximate) square of its speed. Working backwards, the reverse is also true, namely that a body's air resistance DECREASES as the square root of its reduction in speed. For example, if a body's speed is reduced to 1/10th of its original speed, its air resistance will be approximately 1/100 times less than its original air resistance.

The factors that affect air resistance of high speed projectiles are the same factors that affect air resistance of a ball rolling down a track. The difference is simply that your ball rolling down a track, is moving far too slowly to generate any significant are resistance.

Let us take your projectile-versus-rolling ball comparison for example. Let us suppose your ball-shaped projectile is moving at 1000 meters per second, and your rolling ball is moving at 1 meter per second. That is a 1000-to-1 difference in speed, which means that the ball encounters approximately 1 million times (1000 squared) less air resistance than the same size and shaped projectile! Clearly, you can see why objects traveling at slow speeds, have insignificant values of air resistance.

Regards, Bob Wilson


Hi Allison,

Thanks for the question. The same concepts about how air resistance act on a projectile also apply to a rolling object. You are correct in that the speed squared and the shape of the object are major factors in determining the air resistance. Not to be forgotten are the effect of the object rolling. This rolling effect can actually decrease the air resistance--this is the reason that some types of golf balls travel further than expected.

I hope this helps answer your question. Thanks Jeff Grell


I am trying to learn how air resistance might affect a rolling ball, say, a marble, but most of what I find deals with projectiles. Does air resistance on an object change if the object is on a track instead of in the air? Do the factors affecting air resistance on projectiles affect air resistance on a ball in a track in the same way? (Main factors being velocity squared and size and shape of the object.)

Your question is more complicated than I appreciated. There are many variations. A Google search on the term: [ air resistance rolling ball ] found

This site:

http://en.wikipedia.org/wiki/Rolling_resistance#Railroad_steel_wheels_.28torque.29

There are a lot of conditions depending on the materials involved, the rolling friction, etc.

Vince Calder



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